For example, a brushless motor is employed as a blower motor for a vehicular airconditioner. For example, pulse width modulation control (PWM control) is known as a control method for controlling rotation speed of the brushless motor. The PWM control is implemented by activating and deactivating a field-effect transistor (FET) for supplying a driving current to a motor thereby to output a PWM signal in a pulse shape. Thus, the duty ratio of the PWM signal is modified to control rotation of the motor.
The present applicant discloses a PWM control for a brushless motor in publication of Japanese patent 3854186 in which a duty ratio of a PWM signal is gradually reduced in the end of electricity supply to each coil of the brushless motor. Specifically, a slope section is added to the end of the electricity supply gently to switch electricity supplied to the coil thereby to cause the electricity supplied to each coil of the brushless motor in an asymmetric waveform.
For example, a motor, such as a brushless motor, may be equipped with a protection circuit for detecting an electric current flowing into the motor and for terminating electricity supply to avoid excessive electric current flow when detecting excessive electric current flow into the motor.
An electric current, which flows into a motor, changes gently as compared with a PWM control signal, which changes stepwise. Therefore, in a case where an electric current flowing into a motor is detected at a detection time point at an intermediate time point in an activation period of a PWM control signal, a sample hold may be caused in the course of increase in an electric current.
In consideration of this, the present applicant discloses a configuration for detecting a motor current value of a brushed DC motor with high accuracy in publication of U.S. Pat. No. 7,183,737 B2 (publication of Japanese patent application No. 2005-51993). Specifically, in the detection configuration, an electric current flowing into the motor is detected in a detection period between an edge time point of an edge of the PWM signal and a time point in advance of the edge time point. At the edge of the PWM signal, the level of the PWM signal changes to deactivate an FET, which is activated to supply a driving current to the brushed DC motor.
It is noted that, the art of the publication of Japanese patent application No. 2005-51993 is for a brushed DC motor and may not be applicable, as it is, to the configuration of the publication of Japanese patent 3854186 in which the slope period is added to the end of the electricity supply to each coil of the brushless motor in order to decrease gradually the duty ratio of the PWM signal. That is, in such a combination, an electric current flowing into the motor may be undetectable steadily in the slope.